SEOUL, South Korea >> Key North Korean websites were back online Tuesday after an hours-long shutdown that followed a U.S. vow to respond to a crippling cyberattack on Sony Pictures that Washington blames on Pyongyang.

The White House and the State Department declined to say whether the U.S. government was responsible for the Internet shutdown in one of the least-wired and poorest countries in the world.

Though it denies responsibility for the Sony hack, Pyongyang has called it a “righteous deed” and made clear its fury over “The Interview,” a comedy that depicts the assassination of the North’s authoritarian leader, Kim Jong Un, the head of a 1.2 million-man army and the focus of an intense cult of personality.

South Korean officials, speaking on condition of anonymity because of office rules, said the North’s official Korean Central News Agency and the Rodong Sinmun newspaper, which are the main channels for official North Korea news, had earlier been down. But the websites were back up later Tuesday. Among the posts glorifying the ruling Kim family was one about Kim Jong Un visiting a catfish farm.

U.S. computer experts described the Internet outages in the North as sweeping and progressively worse. Jim Cowie, chief scientist at Dyn Research, an Internet performance company, said in an online post that the North came back online after a 9 ½-hour outage.

Possible causes for the shutdown include an external attack on its fragile network or even just power problems, Cowie wrote. But, he added, “We can only guess.”

Last year, North Korea suffered similar brief Internet shutdowns of websites at a time of nuclear tensions with the U.S., South Korea and other countries. North Korea blamed Seoul and Washington for the outages.

President Barack Obama has said the U.S. government expected to respond to the Sony hack, which he described as an expensive act of “cyber vandalism” by North Korea.

Obama did not discuss details, and it was not immediately clear if the Internet connectivity problems represented the retribution. The U.S. government regards its offensive cyber operations as highly classified.

But U.S. options for acting against North Korea are limited. The country already faces massive international and U.S. sanctions over its repeated nuclear and rocket tests.

The hack has been a nightmare for Sony, which canceled plans to release the movie after a group of hackers made threats against theaters that planned to show it.

Although North Korea is equipped for broadband Internet, only a small, approved segment of the population has any access to the World Wide Web. Few North Koreans have access to computers; those who do are typically able to connect only to a domestic Intranet.

More than a million people use mobile phones in North Korea. The network covers most major cities, but users cannot call outside the country or receive calls from outside. The North’s Intranet gives access to government-sanctioned sites and works with its own browsers, search engine and email programs, according to South Korea’s Unification Ministry.